Isocyanates are produced in large quantities and serve mainly as starting materials for the preparation of polyurethanes. They are usually prepared by reaction of the corresponding amines with phosgene, with phosgene being used in a stoichiometric excess. The reaction of the amines with the phosgene can be carried out either in the gas phase or in the liquid phase. In these syntheses, the excess phosgene is generally obtained at least partly together with the gaseous by-product hydrogen chloride liberated in the reaction, so that separating off the excess phosgene from the by-product hydrogen chloride and recirculating it to the reaction is indispensable for economical operation of an isocyanate synthesis. Particularly when the phosgene stream to be recirculated is divided over a plurality of reactors, ensuring the intended phosgene flow to each individual reactor, especially under non-steady-state operating conditions, represents a special technical challenge, so that ensuring a sufficient and stable pressure in phosgene production is of considerable importance.
Various processes for preparing isocyanates by reaction of amines with phosgene in the gas phase are known from the prior art.
GB 737 442 describes a process for recovering liquid phosgene from gas mixtures containing hydrogen chloride and phosgene, in which the gas mixture flows upward through a cooler which is cooled to from −40 to −60° C., with the phosgene condensing and running down into a stock tank. This document states that the recovered liquid phosgene can, owing to its low content of hydrogen chloride of less than 0.7% by weight, be used without further purification in reactions with amines. However, it gives no information as to how the recovered liquid phosgene can be used in an economically advantageous way in a gas-phase reaction. In addition, the process disclosed has the disadvantage that the hydrogen chloride gas leaving the cooler still contains appreciable amounts of phosgene which are thus lost to the phosgenation reaction. A further disadvantage is that the condensation is carried out at a very low and thus energy-consuming temperature level.
U.S. Pat. No. 2,764,607 describes a process for recovering phosgene from a gas mixture with hydrogen chloride from the production of chloroformates. For this purpose, the phosgene/hydrogen chloride gas mixture leaving the condenser mounted on the reaction vessel is firstly brought into contact with cold solvent, with the phosgene being preferentially absorbed in the solvent. The absorbed phosgene together with the partially also absorbed hydrogen chloride is then continuously separated off again from the solvent in a distillation column. For this purpose, the feed is introduced between stripping section and enrichment section and a runback to the distillation column, which completely frees the gas mixture taken off at the top of solvent, is produced by means of an overhead condenser. The phosgene is completely liquefied and separated off from the gas stream obtained and is fed to a storage container. A disadvantage of the process disclosed is the high demand for cooling power at a low temperature level.
An alternative method of fractionating gaseous mixtures composed of hydrogen chloride and phosgene is described in DE 102 600 84. The document discloses a process in which the phosgene is condensed under superatmospheric pressure and the condensed phase is stripped to remove the hydrogen chloride in a subsequent process step. The stripping is necessary since appreciable amounts of hydrogen chloride dissolve in the condensate because of the superatmospheric pressure and, according to the teaching of the document, these have a disadvantageous effect in phosgenation reactions. A disadvantage of the process disclosed is that, owing to the prevailing condensation pressure, a further process step for separating off the dissolved hydrogen chloride is necessary. The document gives no information on the recovery of gaseous phosgene. The document states that the hydrogen chloride/phosgene separation can be carried out under a high pressure, but this increases the safety risks. In addition, the generation of high pressure is energy-consuming. As an alternative, a description is given of a separation at very low temperatures, but this is likewise energy-consuming and in addition leads to high contents of hydrogen chloride in the liquid, phosgene-containing phase.
U.S. Pat. No. 3,544,611 describes a process for preparing organic isocyanates. The liquid reaction solution is fed into the middle part of a distillation column in order to take off hydrogen chloride at the top and phosgene and isocyanate at the bottom. The bottom stream is fed to a further distillation column in order to separate the isocyanate and phosgene from one another and recirculate phosgene to the reaction. A disadvantage of the process described is the high pressure of 10-50 bar at which the distillation has to be operated in order to be able to separate the mixtures by distillation at economically advantageous coolant temperatures.
The abovementioned documents do not disclose specific instructions relating to technical procedures allowing excess phosgene to be recovered in a very economical way and recirculated to the phosgenation reaction. As a result, the processes lose economical attractiveness.
WO 2007/014 936 discloses a process for preparing diisocyanates by reaction of diamines with a stoichiometric excess of phosgene in the gas phase, with the excess phosgene being at least partly recirculated to the reaction and the phosgene stream to the reactor containing less than 15% by weight of hydrogen chloride before mixing with the amine. According to the teaching of this document, this is said to improve the time of operation of the reactors since precipitation of amine hydrochlorides is said to be reduced. A disadvantage of such high contents of inert hydrogen chloride gas in the phosgene gas is that it leads, as a result of increased recycle streams, to an increase in operating costs and also to large apparatuses and thus high plant construction costs. In an embodiment described, the excess phosgene and the hydrogen chloride formed are firstly separated off from the essentially gaseous reaction mixture and the excess phosgene is then at least partly recirculated to the reaction, with hydrogen chloride being separated off from this recirculated phosgene in such a way that the phosgene stream contains less than 15% by weight of hydrogen chloride before mixing with the amine stream. No information is given regarding the content of solvent in the recirculated phosgene stream. The document teaches that the separation is preferably carried out by means of a combination of a distillation and a scrub. Here, the phosgene is scrubbed out of the hydrogen chloride-containing stream by means of a scrubbing medium. The separation of the phosgene and the hydrogen chloride from this loaded scrubbing medium is preferably carried out by distillation. The scrub and the distillation can, according to the description, be operated at pressures of from 1 to 10 bar absolute. Further details regarding the separation of phosgene from the loaded scrubbing medium are not disclosed by the document.
According to the teaching of WO 2008/086 922, the phosgene must not contain more than 1000 ppm by weight of chlorine before mixing with the amine in a gas-phase phosgenation reaction since otherwise the risk of materials embrittlement would arise because of the high temperatures. According to this teaching, a certain amount of chlorine is always formed because of the dissociation of phosgene at high temperatures, so that removal of this chlorine is necessary. In addition, the document discloses an embodiment in which the gas mixture containing phosgene, hydrogen chloride and chlorine is firstly subjected to a partial condensation (page 18, line 30) and scrub (page 19, line 18). Here, a liquid phase containing phosgene, scrubbing medium, hydrogen chloride and chlorine is obtained. This is then freed of the low boilers chlorine and hydrogen chloride in a first rectification (referred to as c)). In a subsequent step, phosgene and scrubbing medium are separated from one another in a second rectification (referred to as e)) (page 20, line 26 to page 21 line 11). The document discloses two embodiments of the second rectification column: in the first, the rectification column has only a stripping section so that the overhead product is taken off without purification via separation-active internals at the top. The characterization of the composition of the bottom product is not clear; it is recirculated to the phosgenation reaction. Nothing is said about the further use of the low boiler stream eL.
In the preferred second embodiment, the second rectification additionally has an enrichment section which, at an appropriate reflux ratio, makes it possible for the overhead stream to consist essentially of pure phosgene which can be used without further purification in the phosgenation. In this embodiment having an enrichment section, the bottom stream consists essentially of pure scrubbing liquid.
WO 2009/037 179 discloses a process for preparing isocyanates in the gas phase, in which the freshly produced phosgene is introduced without prior condensation into the gas-phase reaction. As a result of the apparatus and energy for phosgene condensation, intermediate storage of liquid phosgene and phosgene vaporization being able to be dispensed with, the phosgene holdup in the plant is reduced and the energy for vaporizing the phosgene is saved (page 5, lines 32-42). A disadvantage of this process is the lack of opportunity of separating off accompanying components present in the fresh phosgene, which otherwise contaminate the hydrogen chloride stream discharged from the process.
Furthermore, this document describes the method of separating phosgene from a gas mixture with hydrogen chloride and recirculating the phosgene which has been separated off to the gas-phase phosgenation by means of a combined scrub and multistage distillation.
On the subject, the document states that a scrubbing liquid loaded with phosgene and hydrogen chloride is firstly obtained in a first step by scrubbing of the phosgene/hydrogen chloride gas mixture with a scrubbing liquid. This is followed by a first distillation step in which the hydrogen chloride is very largely removed from the phosgene-containing scrubbing solution and is returned to the preceding scrubbing step, followed by a second distillation step in which the previously obtained scrubbing solution is separated into gaseous phosgene and very largely phosgene-free scrubbing liquid. The gaseous phosgene is introduced into the gas-phase phosgenation, while the scrubbing liquid is reused for scrubbing of the phosgene/hydrogen chloride gas mixture. The document does not disclose how the phosgene-scrubbing liquid separation is configured in terms of apparatus or the purity of the recirculated phosgene stream which can be achieved.
According to the general teaching of this document, a two-stage distillation process is consequently the process of choice in order to recover gaseous phosgene from a phosgene-laden scrubbing medium for recirculation to the phosgenation reaction. The omission of the condensation and storage of fresh phosgene is the only specific measure named by the document for achieving a lower phosgene holdup. Although a process consisting of two distillation steps (and thus the use of separation apparatuses having a significant phosgene holdup) are proposed as preferred embodiment, no details regarding the configuration in terms of apparatus of the two distillation steps are given, in particular not in respect of a configuration in terms of apparatus which minimizes the phosgene holdup.
WO 2011/003 532 discloses a process for preparing isocyanates by reaction of primary amines with phosgene in a stoichiometric excess in the gas phase, in which the excess phosgene is subsequently recovered and recirculated to the reaction. The recovery of phosgene from the gas mixture containing phosgene and hydrogen chloride is carried out in two stages. In the first step (hydrogen chloride-phosgene separation), the gas mixture containing hydrogen chloride and phosgene which leaves the reactor is separated into a gaseous stream containing mainly hydrogen chloride and a liquid stream containing phosgene and the liquid stream previously obtained is converted in a second step (phosgene gas production) into a gaseous, phosgene-containing stream, wherein the pressure in the first process step is lower than the pressure in the second process step. The process is advantageous since it allows the recovery of phosgene from the liquid, phosgene-containing scrubbing medium solution in only one step (phosgene gas production). In a preferred embodiment, phosgene gas production is carried out in a distillation column having 2-45 theoretical plates. The column can contain a stripping section and/or enrichment section; the column preferably contains both a stripping section and an enrichment section, with the feed stream preferably being introduced between stripping section and enrichment section. The column is preferably operated at a temperature at the bottom of 140-220° C.
In a preferred embodiment, the column is provided with an overhead condenser. The overhead condenser is preferably operated at a coolant entry temperature of from −25 to 0° C. The condensate produced by means of the overhead condenser can be partly or entirely recirculated to and/or taken off from the column; the condensate is preferably recirculated in its entirety to the column.
Disadvantages of the preferred embodiment are the large dimensions of the column for phosgene gas production and also the expensive use of cold for producing a runback stream.
In an alternative embodiment, phosgene gas production is carried out by the phosgene-containing liquid stream from the hydrogen chloride-phosgene separation being separated by partial vaporization into a gaseous stream containing phosgene and possibly inerts and a liquid stream. For this purpose, the liquid stream obtained from the hydrogen chloride-phosgene separation is fed into an evaporator whose temperature at the bottom is preferably 100-220° C. Compared to the abovementioned preferred embodiment, this embodiment reduces the apparatus and refrigeration energy costs and also the liquid phosgene holdup, but allows only low purities of the gaseous phosgene stream produced and of the stream which contains predominantly scrubbing medium and is discharged in liquid form from the bottom of the evaporator.
In addition, the document teaches that, in order to optimize the energy consumption, the phosgene-containing liquid stream from the hydrogen chloride-phosgene separation is preferably conveyed indirectly to phosgene gas production, i.e. is fed to phosgene gas production after particularly preferably heating to 5-175° C. by heat exchange with other process streams.
According to example 5 in WO 2011/003 532, phosgene gas production is configured in the form of a stripping column, i.e. without enrichment section. (In distillation technology, the term enrichment section refers to the part made separation-active by means of internals above the inlet of a distillation column. The enrichment section increases the purity of the overhead product. In a stripping column, the stream to be distilled is introduced at the top of the distillation column, so that a stripping column does not have an enrichment section.) The phosgene solution obtained from the hydrogen chloride-phosgene separation is pumped directly, i.e. without prevaporization, at a temperature below 10° C. to the top of the stripping column. Gaseous phosgene containing about 0.2% by weight of solvent is taken off at the top of the stripping column. However, this embodiment has the disadvantage that, owing to the low temperature at the top of the desorption column, no energy-saving heat integration by preheating and partial prevaporization can be achieved. Furthermore, the phosgene solution can have different compositions and temperature because of varied process parameters (including phosgene excess in the reaction, reactor outlet temperature, amount of solvent for the quench, amount of solvent for phosgene absorption) and because of process fluctuations. When introduction occurs directly via the top of the stripping column, this inevitably leads to an altered and possibly fluctuating composition and temperature of the phosgene return stream to the reaction, since a regulable overhead condenser which has a stabilizing effect is not available in this embodiment. Fluctuating conditions of the phosgene return stream can lead to a lower energy efficiency, reduced reaction yield, increased operating costs and reduced availability.
WO 2011/003 532 mentions the possibility of hydrogen chloride-phosgene gas mixtures from a plurality of reaction lines being treated in a single hydrogen chloride-phosgene separation. However, it is not disclosed how the sufficiently stable column pressure which is essential for parallel operation of a plurality of reactors can be ensured even in the case of quick operating point changes. Since gas-phase reactors have a narrow favorable load range, a quick operating point change is advantageous, for example, on starting up and running down individual reactors.
A series of procedures by means of which mixtures of hydrogen chloride and phosgene and also possibly solvent can be fractionated in order to allow recirculation of the phosgene into the reaction are thus known from the prior art. However, the prior art does not go into detail regarding the problems of making phosgene recirculation controllable by means of regulation even under relatively difficult conditions. Such relatively difficult conditions can be, for example, unexpected process fluctuations, for example triggered by fluctuations or brief interruptions in individual parts of the plant, e.g. in fresh phosgene production. However, planned alterations to the process can also become challenges in terms of regulation. This is, in particular, the case when a gas-phase phosgenation is to be carried out in a plurality of reactors connected in parallel, the gaseous reaction products of which are worked up in a joint work-up section.
The parallel operation of a plurality of manufacturing apparatuses having the same configuration is nothing unusual in the industrial production of chemical products. The division of a desired total production capacity over a plurality of manufacturing apparatuses (production lines or trains) operated in parallel is advisable when, inter alia,                The construction of the apparatuses allows a smaller maximum production capacity than the desired total capacity of the production plant (cf. “Rules of Thumb for Chemical Engineers”, 4th Ed., 2005, page 242, chapter “Process Evaluation—What Size should a plant be?”)        Partial processes or apparatuses have to be shut down occasionally for cleaning purposes, e.g. because of a tendency to suffer from fouling, without total production being shut down for this purpose (Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2012, chapter “Chemical Plant Design and Construction”, volume 8, page 267)        Partial processes or apparatuses have a higher risk of going down than is acceptable for the total plant (Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2012, chapter “Chemical Plant Design and Construction”, volume 8, page 273)        Partial processes or apparatuses have a more restricted possible load range than is to be made possible for the total plant.        
EP-A-570 799 states that gas-phase reactors for the phosgenation of amines have only a narrow favorable load range and that solids formation can occur in the reactors, making interruptions to production for cleaning purposes necessary. For these reasons, inter alia, the installation of a plurality of reaction lines for phosgenation which can be operated in parallel can be advantageous. To reduce the apparatus costs, it can, on the other hand, be advantageous nevertheless to configure upstream and/or downstream process steps (e.g. phosgene production, amine vaporization, hydrogen chloride-phosgene separation) jointly for all reaction lines.
EP-A-2 196 455 describes, inter alia, an embodiment of a production process for isocyanates in which the crude product is fed to two reaction lines having a joint reaction termination zone (quench).
If a joint phosgene gas production is operated so as to feed the recovered phosgene stream in gaseous form to a plurality of reaction lines, phosgene gas production and the reactors are connected to one another via a gas space, so that a change in the process pressure in one apparatus acts on the connected apparatuses. This applies particularly when there are no actively regulable pressure-increasing or pressure-reducing elements in the connection between phosgene gas production and reactors. Sudden changes, in particular, in process parameters (e.g. safety shutdown or load change in a reaction line) lead to undesirable instabilities in the total plant, possibly through to the need to take the total plant out of operation. An important parameter for trouble-free and stable operation of the reaction is ensuring of a constant phosgene excess in the phosgenation reaction. In order to ensure a constant stream of gaseous phosgene, a constant supply of pressure of the phosgene gas source, i.e., for example, a distillation column, is essential. The regulation of the pressure in this distillation column is therefore of substantial importance for the total process stability.
In order to lower the apparatus costs and, taking into account the increasing desire for safety, to minimize the phosgene holdup, it would be advantageous to be able to dispense with the enrichment section and possibly also the overhead condenser of the distillation column used for phosgene gas production. However, the omission of the overhead condenser makes pressure regulation more difficult. In addition, the prior art does not disclose any process which would combine the omission of the enrichment section with an energy-efficient preheating/prevaporization of the feed to phosgene gas production.
The prior art does mention that an inert material, e.g. solvent, can be introduced into the gas-phase reaction, with the range of the possible solvent contents described in the phosgene return stream extending up to 10 mass 90. However, in all specific working examples disclosed which describe the recovery and recirculation of phosgene used in excess back into the gas-phase reaction, a very low solvent content in the phosgene stream is nevertheless obviously sought in practice (see, for example, the document WO 2011/003 532 discussed comprehensively above: according to this, only a few ppm of solvent are permitted in order to minimize the energy usage; in example 5, mention is made of a solvent concentration of 0.2%, which is the maximum value disclosed in a specific working example). This is achieved by either the phosgene-solvent separation column being equipped with an enrichment section and being operated with runback or by a low temperature level being maintained at the top of the column (in WO 2011/003 532, a temperature of the feed to the top of the column of not more than 10° C. is mentioned).
The prior art accordingly does not give any information as to how the advantages of                a. phosgene gas production without enrichment section (lower apparatus costs, lower phosgene holdup, lower condensation energy costs) and        b. energy-efficient heat integration for feed preheating and/or vaporization can be combined with one another.        
There was therefore a need for a process which is very simple in terms of apparatus and favorable in terms of energy for preparing isocyanates, which keeps the phosgene holdup very low without thereby making the recirculation of the excess phosgene to the reaction more difficult, in particular under conditions which are challenging in terms of regulation (e.g. the use of a plurality of reaction lines operated in parallel with joint work-up).